Unexplained joint swelling may require aspiration to aid diagnosis. The aspiration itself may be of therapeutic value lowering the pressure and relieving pain. It is often coupled with intra-articular washout or instillation of steroid or antibiotic as appropriate. Examination of the synovial fluid may be of diagnostic value (see Table 8.2).
Crystal arthropathies such as gout and pseudogout can be differentiated on examination of the crystals with polarised light microscopy. The crystals of gout are negatively birefringent, whereas the crystals of pseudogout are positively birefringent.